1. Field of the Invention
The present invention relates to an envelope detection apparatus for a video cassette recorder (VCR). The present application is based upon Korean Application No. 96-2090,which is incorporated herein by reference.
2. Description of the Related Art
A conventional VCR detects an amplitude of a video signal read out from a video tape, that is, an envelope. When an envelope is detected not more than a predetermined level for a certain time or more, one VCR judges that there are no signals recorded on the video tape, and displays only a blue color background on a monitor. When an envelope is detected not more than a predetermined level for a certain time, the other VCR judges that a video signal is dropped out and compensates the drop-out portion using a previous video signal. As described above, it is essential to detect an envelope in VCR technology since an envelope detection result is used for other functions of the VCR.
An operation of an envelope detection apparatus for a conventional analog VCR shown in FIG. 1 will be described below with reference to FIGS. 2 and 3A through 3H. In FIG. 1, a playback amplifier 11 receives a video signal read out from a video tape via a playback head (not shown), amplifies the received video signal by a predetermined amplification factor, and outputs the amplified signal shown in FIG. 3A to a rectifier 12. The rectifier 12 rectifies the input signal and outputs the rectified signal shown in FIG. 3B to a smoothing portion 13. The smoothing portion 13 smooths the input signal. The smoothed signal Vs shown in FIG. 3C is input to a non-inverted input terminal of a comparator 14 shown in FIG. 2. A predetermined reference value V.sub.I is input to an inverted input terminal of the comparator 14. Here, the reference value V.sub.I is pre-set based on an ordinary noise level. The comparator 14 compares the signal Vs with reference value V.sub.I and generates a rectangular wave shown in FIG. 3D representing an envelope detection result.
The above-described envelope detection apparatus cannot obtain a desired result when an output level of the playback amplifier is varied due to factors such as a state of a signal recorded on a video tape, an output error according to temperature variation of the playback amplifier and a state of a head. For example, when an amplification factor of the playback amplifier 11 is enlarged due to the above-described factors, the output signal Vs of the smoothing portion 13, has a value more than the reference value V.sub.I,on the whole, as shown in FIG. 3E. As a result, the width of the envelope detection signal is increased, as shown in FIG. 3F. Also, the output signal Vs of the smoothing portion 13 when the amplification factor of the playback amplifier 11 is decreased is shown in FIG. 3G, in which case the width of the envelope detection signal is decreased, as shown in FIG. 3H.
The envelope detection apparatus of Japanese laid-open patent publication No. Heisei 7-121983 filed by Matsushida solves this problem by using only a constant reference value, in spite of variation of the amplification factor of the playback amplifier 11. FIG. 4 shows this apparatus and FIGS. 5A through 5D show waveform diagrams used for explaining the operation of the FIG. 4 apparatus.
In FIG. 4, since the operations of a playback amplifier 11, a rectifier 12, and a smoothing portion 13 are the same as those of the corresponding blocks having the same reference numerals in FIG. 1, the description thereof will be omitted. FIG. 5A shows an output waveform of the playback amplifier 11, and FIG. 5B shows an output waveform of the rectifier 12. A peak detector 24 detects a peak level "b" from an output signal "a" of the smoothing portion 13, as shown in FIG. 5C. A leak portion 25 reduces the peak level "b" by a predetermined level and outputs a signal "c" shown in FIG. 5C. A reference voltage setter 26 sets the output signal "c" of the leak portion 25 to be the reference voltage when the output signal "c" of the leak portion 25 is smaller than the set reference voltage, and prevents a detection error in the absence of a signal. A comparator 27 compares the output signal "a" of the smoothing portion 13 with the reference voltage output from the reference voltage setter 26, and outputs the comparison result as an envelope detection result shown in FIG. 5D. Therefore, a constant envelope detection result can be obtained even though the output level of the playback amplifier is varied. Meanwhile, a speed setter 28 varies a filter characteristic of the smoothing portion 13 and a time constant of the leak portion 25 according to a speed variation, thereby enabling detection of an envelope during a speed reproduction which is different from a normal speed reproduction, for example, a multiple speed reproduction.
However, actual envelope detection is affected by the circumstances of the whole system, and an analog VCR can most accurately detect an envelope when considering a carrier-to-noise (C/N) ratio, while a digital VCR can when considering a bit error rate (BER). In other words, although the amplification factor of the playback amplifier is not the same, an identical envelope can be detected if a C/N ratio or a BER is the same.
In the above prior art, the envelope detection relies on only the amplitude of the output signal of the playback amplifier. Thus, when an amplitude difference of a signal output from the playback amplifier is larger, an envelope cannot be more accurately detected. Furthermore, the speed setter 28 uses a time constant corresponding to each speed in order to detect envelopes at various reproduction speeds. As a result, a circuit for setting such a time constant is complicated and a separate circuit is needed for selecting a corresponding time constant.